A line head includes light-emitting element lines formed by arranging a plurality of light sources in a line shape in a main scanning direction. Each of the light sources is turned on/off corresponding to image data. Light beams emitted from the light sources pass through a lens array to form imaged spots on an exposed surface. The imaged spots generated by making the light beams emitted from the plurality of light sources imaged on the exposed surface are shifted by inches in the main scanning direction or a sub-scanning direction so as to overlap each other, thereby forming images. A gray-scale image of the images has a screen structure displayed on the basis of an area of dots or lines having a predetermined pitch. A diameter of each of the imaged spots formed on the exposed surface is set to be larger than a pitch between pixels and smaller than a pitch between lines or dots forming the screen. Gradation of an image is displayed by a combination of binary states of ON/OFF of each of the light sources.